1. Field of the Invention
The present invention relates to a semiconductor device with circuits constructed of thin-film transistors (referred to TFT hereinafter). More particularly, the present invention relates to electro-optical equipment typified by liquid crystal display devices and EL (electroluminescence) display devices, and to semiconductor circuits. The present invention relates also to electrical equipment (or electronic equipment) with the electro-optical equipment or semiconductor circuits according to the present invention.
The term “semiconductor device” as used in this specification embraces any and all devices which function by utilizing the characteristics of semiconductors. They include electro-optical equipment, semiconductor circuits, and electrical equipment.
2. Description of the Related Art
By virtue of its ability to be formed a transparent substrate, the thin film transistor (referred to as TFT hereinafter) has been actively investigated for its application to the liquid-crystal display of active matrix type (referred to as AM-LCD hereinafter). On account of its high mobility, the TFT made with crystalline semiconductor film (typically polysilicon film) is expected to display fine images if functional circuits are integrated on the same substrate.
AM-LCD is basically constructed of a pixel portion (or pixel matrix circuit) to display images, a gate driving circuit (or gate driver circuit) to drive TFT for each pixel arranged in the pixel portion, a source driving circuit (or source driver circuit) to send image signals to TFT for each pixel, and a data driving circuit (or data driver circuit), all of which are formed on the same substrate. Incidentally, the region in which the gate driving circuit and source driving circuit are formed is called the driving circuit portion.
Recently, the system-on-panel has been proposed which has, in addition to the pixel portion and driving circuit portion, signal processing circuits, such as signal driving circuits and γ-correction circuits, formed on the same substrate.
However, it is difficult to meet all the circuit requirements with TFT of the same structure because the pixel portion and the driving circuit portion differ from each other in their performance required. In other words, the driving circuit portion containing shift register circuits attaches importance to high operation, while the TFT constituting the pixel portion (referred to as pixel TFT) attaches importance to the high withstanding voltage. So far, there has been no TFT structure to satisfy both of them.
The present applicant filed an application for construction which is characterized in that the TFT constituting the driving circuit portion (referred to as driving TFT hereinafter) and the pixel TFT differ from each other in the thickness of the gate insulating film. (Japanese Patent Laid-open No. 10-056184 and its corresponding to U.S. patent application Ser. No. 08/862,895). To be concrete, the gate insulting film of the driving TFT is made thinner than the gate insulating film of the pixel TFT.